Costs involved in a wind turbine tower structure are generally of the order of about 20-25 percent of the total costs of the wind turbine, depending on tower height and power of the wind turbine. It is therefore quite important for the final wind turbine costs to build towers as optimally as possible. Development of increasingly higher wind turbines (above 80 m) entails high loads in the structural parts to be taken into consideration. High bending moments and loads caused by wind thrust requires tower sections diameter and wall thickness to be increased. In practice, however, tower dimensions are limited (up to 4.5 m in diameter) due, for example, to transport requirements. Such a limitation results in a non-linear increase of tower costs as the wall thickness is increased and hence the tower design obtained is not optimal. Furthermore, other limitations are involved in tower designs such as tower wall thickness due to manufacturing processes.
The use of large diameter wind turbine towers can be achieved through the use of towers formed with shell segments and connecting means for joining the segments to each other. Shell segments, when connected, define a closed a tower section and several tower sections connected to each other (arranged one above the other) define the wind turbine tower structure.
One example of such a construction is disclosed, for example, in WO2007095940 which relates to a wind turbine tower comprising two or more adjacent shell segments connected to one another through plates and bolts.
WO2004083633 provides a windmill steel tower comprising a number of tower sections each comprising two or more elongated shell segments, which combine into a complete tower section by means of vertical flanges tightened together by bolts.
The main disadvantages with these solutions relate to high manufacturing, installation and maintenance costs due to the large number of pre-stressed bolts required at vertical flange connections.